Portions of coaxial telephone cables laid at great depths are constituted by a central twisted core made of wires of hard high tensile steel, a copper tube welded longitudinally and tightly applied about the core to constitute the inner conductor of the coaxial cable, a dielectric layer which is extruded and centred, a metal tube tightly fitted about the dielectic layer to constitute the outer conductor of the coaxial cable and lastly a plastics sheath extruded onto the outer conductor.
Such a cable which is laid at great depths, includes no other element such as an outer armour for example and must therefore withstand high stresses, by means of the central core of high-tensile steel, as for example, when it is lifted.
The strength of the core must be sufficient to lift the cable when it is laid at great depth; the maximum length of cable suspended in the water before the top end breaks is called the modulus. It is expressed by the formula:
Modulus=Breaking strength (tonnes)/Weight in water (tonnes per nautical mile)
The DC electrical resistance is a fundamental parameter of long-distance connections. Indeed, the AC electric equipments strung out along the cable are fed in series from the terminal stations with DC by the central conductor. High ohmic voltage drops in the line could lead to feed voltages at the terminal stations which would be too high for the system to be reliable.
The compound (steel-copper) central conductor is manufactured in a single operation by a machine constituted mainly by:
a linear twisting machine,
a tube forming machine,
a welding unit,
a tightening unit.
All these elements are placed in line and operate in synchronism.
Twisting machine:
A tube contains as many bobbins as there are wires to be assembled, suspended about its axis of rotation and of symmetry. Said bobbins unwind, turning only round their own axis of rotation. The tube rotates at a speed proportional to the pulling speed of the core. The wires are guided along the walls of the tube and at its end by ramps or guides made of metal or a ceramic substance. Their paths then converge at the end of the tube towards a die whose size governs the diameter of the core and the continuity of its pitch. The wires are generally braked by small cords rubbing on a groove on the cheek of each bobbin.
Tube forming machine:
Motor-driven rollers whose speed is proportional to the pulling speed form the tube round the core from a sheet which has previously been cut to the exact width.
Welding unit:
The lips of the tube are welded lengthwise by a conventional inert gas arc welding method.
Tightening unit:
The inside diameter of the welded tube is firstly reduced to a diameter close to that of the core, by a reduction in its cross-section without any reduction in its thickness, by means of motor-driven rollers whose speed is proportional to the pulling speed.
Then, the drawn down tube and the core are forced through a tungsten carbide tightening die, which causes a penetration of the copper between the wires and which calibrates the diameter of the inner conductor.
It was also proposed in U.S. Pat. No. 3,834,149 to assemble the steel wires of a wire rope with wires or a foil of aluminium or zinc, in order to protect the steel wires against corrosion.